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"Unable to instantiate prefab. Prefab may be broken"

Hey there ! I just upgraded my project from 2017.3 to 2018.1 and now all of my scenes are broken (missing prefab for every object) and when I try to drag'n drop a prefab into the scene I get the following error :

I am having the same issue... I upgraded to unity 2018 , as want to use unity inbuilt TextMesh Pro. and updated all UI, however now my prefab is not working as it suppose to, having same error as mention. Prefeb is too complex as its a player and enemy with animation, AI and UI... will be pain to recreate same :( is ther any solution to fix it... please help...

I know this is probably useless for most of the people but...since I still had installed Unity 2018 and a its version of the project, I opened it, broke all prefab's instances and saved. Then in Unity 2017 I opened that version and recreated all the prefabs (hopefully they were just a few and I had them all in my scenes). Still waiting for an answer though.

I get this problem often its so annoying. I downloaded an asset other day and tried it. Everything went fine and i didnt touch that asset folder in my project since then was working on something else. Then i come back couple days or week later and try to place a prefab on the scene bam broken. Nothing changed at all. Unity often randomly give bug like this using 2018.1.1f1. It happened with many other asset. When its not its a script in my scene that get unlinked for some reason. I think i lost a whole week since 2 month trying to figure out bugs that was related to broken prefabs or missing script that randomly appear. Is there a known unity version working at best? Im gonna give it a try.

This is most likely the same issue as in this question over here. The problem is that Unity seems to have changed it's prefab serialization format and didn't make it backward nor forward compatible. It's mainly a rename of some fields. If you use text serialization you could rename those fields yourself. However i strongly recommend you create a backup of your project.

Also if you can verify that this is the problem, please file a bug report with as many details possible. So create a new standalone project where you copy that offending prefab to and attach that project to the bugreport. The more people verifying the issue, the faster this may be fixed.

As a general advice: If you are in the middle of a production, do not upgrage Unity. Even Unity doesn't recommend it.

When you upgraded the Unity editor it warned you that it is going to upgrade your project and asked you if you have created a backup. You clicked "yes". If you don't actually have a backup you are the only one to blame.